Non-aqueous electrolytic solution and lithium battery

ABSTRACT

A non-aqueous electrolytic solution comprising a non-aqueous solvent and an electrolyte, which further contains a combination of a nitrile compound and an S═O group-containing compound (or a dinitrile compound) in an amount of 0.001 to 10 wt. % imparts improved cycle performance and storage property to a lithium battery, particularly a lithium secondary battery.

FIELD OF THE INVENTION

This invention relates to a non-aqueous electrolytic solution favorablyemployable for a lithium battery such as a lithium primary battery or alithium secondary battery. The invention further relates to a lithiumbattery showing improved battery performances, particularly, to alithium primary battery having a high energy density and a lowself-discharge ratio and a lithium secondary battery showing good cycleperformance, high electric capacity and good storage endurance.

BACKGROUND OF THE INVENTION

At present, a non-aqueous secondary battery is generally employed as anelectric source for driving small electronic devices. The non-aqueoussecondary battery comprises a positive electrode, a negative electrode,and a non-aqueous electrolytic solution. The non-aqueous lithiumsecondary battery generally comprises a positive electrode of lithiumcomplex oxide such as LiCoO₂, LiMn₂O₄ or LiNiO₂, a non-aqueouselectrolytic solution such as a solution of electrolyte in a carbonatesolvent such as ethylene carbonate (EC), propylene carbonate (PC),dimethyl carbonate (DMC), diethyl carbonate (DEC), or methyl ethylcarbonate (MEC), and a negative electrode of carbonaceous material orlithium metal.

Also known is a lithium primary battery comprising a positive electrodeof, for instance, manganese dioxide and a negative electrode of, forinstance, lithium metal and showing a high energy density.

The non-aqueous secondary battery preferably has good batteryperformances such as large electric discharge capacity and high electricdischarge retention (i.e., good cycle characteristics). However, thereare observed certain problems in the known non-aqueous secondarybattery. For instance, in the non-aqueous lithium ion secondary batteryusing a positive electrode of LicoO₂, LiMn₂O₄, or LiNiO₂, oxidativedecomposition of a portion of the non-aqueous electrolytic solutionundergoes in the electric charging stage. The decomposition productdisturbs electrochemical reaction so that the electric dischargecapacity decreases. It is considered that the oxidative decomposition iscaused in the non-aqueous solvent of the non-aqueous electrolyticsolution on the interface between the positive electrode and theelectrolytic solution.

Moreover, in the non-aqueous lithium secondary battery particularlyusing negative electrode of carbonaceous material of high crystallinitysuch as natural graphite or artificial (or synthetic) graphite,reductive decomposition of the solvent of the non-aqueous electrolyticsolution undergoes on the surface of the negative electrode in thecharging stage. The reductive decomposition on the negative electrodeundergoes after repeated charging-discharging procedures even in thecase of using ethylene carbonate (EC) which is generally employed in theelectrolytic solution.

JP-A-3-289062 proposes to incorporate 0.2 to 10 vol. % of1,4-dimethoxybenzene compound into a non-aqueous solvent comprising ahigh permittivity solvent such as ethylene carbonate (EC) or propylenecarbonate (PC) and a low permittivity solvent such as tetrahydrofuran(THF) so that the cycle characteristics can be improved.

U.S. Pat. No. 5,256,504 and U.S. Pat. No. 5,474,862 propose toincorporate ethyl propionate into a combination of ethylene carbonateand diethyl carbonate (DEC) so that the cycle characteristics can beimproved.

JP-A-9-161845 proposes a lithium secondary battery which employs acombination of a high activity solvent having a donor number of 14 to 20and a low activity solvent having a donor number of 10 or lower. Thispatent publication describes the use of a negative electrode comprisinga carbonaceous material of a graphite crystal structure having a latticedistance (d₀₀₂) of lattice surface (002) of 0.3365 nanometer or more.The patent publication further describes that the high activity solventcan be a cyclic carbonate ester, a cyclic ester, a linear esher, acyclic ether, a linear ether, or a nitrile. The nitrile can be adinitrile such as glutaronitrile or adiponitrile. It is noted that inExample 6 the glutaronitrile is employed in an amount of 19 vol. % in anon-aqueous solvent for preparing a electrolytic solution.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a non-aqueouselectrolytic solution which is favorably employable for producing alithium battery showing improved battery performances such as good cycleperformance, a high electric capacity, good storage endurance, and ahigh electric conductivity.

It is another object of the invention to provide a lithium primary orsecondary battery showing improved battery performances such as goodcycle performance, a high electric capacity, good storage endurance, anda high electric conductivity.

The invention resides in a non-aqueous electrolytic solution comprisinga non-aqueous solvent and an electrolyte, which further contains anitrile compound and an S═O group-containing compound.

The invention further resides in a non-aqueous electrolytic solutioncomprising a non-aqueous solvent and an electrolyte, which furthercontains a dinitrile compound in an amount of 0.001 to 10 wt. %.

The invention furthermore resides in a lithium battery comprising apositive electrode, a negative electrode comprising a carbonaceousmaterial of a graphite crystal structure having a lattice distance oflattice surface (002) of 0.34 nanometer or less and one of theabove-mentioned non-aqueous electrolytic solution of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the non-aqueous solvent employed for producing a non-aqueouselectrolytic solution of the invention, a mononitrile compound or adinitrile compound is contained. The mononitrile compound preferably hasa linear or branched alkyl chain having 1 to 12 carbon atoms which mayhave one or more substituents or an aromatic group and can beacetonitrile, propionitrile, butylonitrile, valeronitrile,hexanenitrile, octanenitrile, undecanenitrile, decanenitrile,cyclohexanecarbonitrile, benzonitrile, or phenylacetonitrile.

The dinitrile compound preferably has a linear or branched alkylenechain having 1 to 12 carbon atoms which may have one or moresubstituents or an aromatic group and can be succinonitrile,glutaronitrile, adiponitrile, 1,5-dicyanopentane, 1,6-dicyanohexane,1,7-dicyanoheptane, 1,8-dicyanooctane, 1,9-dicyanononane,1,10-dicyanodecane, 1,12-dicyanododecane, tetramethylsuccinonitrile,2-methylglutaronitrile, 2,4-dimethylglutaronitrile,2,2,4,4-tetramethylglutaronitrile, 1,4-dicyanopentane,2,5-dimethyl-2,5-hexanedicarbonitrile, 2,6-dicyanoheptane,2,7-dicyanooctane, 2,8-dicyanononane, 1,6-dicyanodecane,1,2-dicyanobenzene, 1,3-dicyanobenzene, or 1,4-dicyanobenzene.

In the invention, the mononitrile compound should be used in combinationwith an S═O group-containing compound.

The dinitrile compound can be used not in combination with the S═Ogroup-containing compound, under the condition that the dinitrilecompound should be contained in the electrolytic solution in such asmall amount as 0.001 to 10 wt. %, preferably 0.01 to 5 wt. %, morepreferably 0.01 to 3 wt. %, most preferably 0.01 to 2 wt. %.

The incorporation of a dinitrile compound into an electrolytic solutionis effective to reduce erosion of a metallic inner surface of a batterycase. If an S═O group-containing compound is incorporated in theelectrolytic solution in combination with a dinitrile compound, theeffect to reduce the erosion of a metallic inner surface of a batterycase becomes more prominent.

The S═O group-containing compound can be a cyclic compound or a linearcompound and can be dimethylsulfite, diethylsulfite, ethylenesulfite,propylenesulfite, vinylenesulfite, dimethylsulfone, diethylsulfone,methylethylsulfone, divinylsulfone, sulforane, sulforene, methylmethanesulfonate, ethylmethanesulfonate, propargyl methanesulfonate,methyl benzenesulfonate, 1,3-propanesultone, 1,4-butanesultone, dimethylsulfate, diethyl sulfate, ethyleneglycol sulfate, or 1,2-propanediolsulfate.

When the electrolytic solution contains a nitrile compound and an S═Ogroup-containing compound in combination, the nitrile compound ispreferably contained in the electrolytic solution in an amount of 0.001to 10 wt. %, more preferably 0.01 to 5 wt. %, more preferably 0.01 to 3wt. %, most preferably 0.01 to 2 wt. %. The S═O group-containingcompound is preferably contained in an amount of 4 wt. % or less, morepreferably in the range of 0.2 to 3 wt. %. The nitrile compound and theS═O group-containing compound are contained preferably in a weight ratioof 1:99 to 99:1, more preferably 9:1 to 1:9, 9:1 to 3:7.

The non-aqueous solvent of the electrolytic solution of the inventionpreferably comprises at least one compound selected from the groupconsisting of a cyclic carbonate, a cyclic ester, a linear carbonate,and an ether.

Preferred examples of the cyclic carbonates include ethylene carbonate(EC), propylene carbonate (PC), butylene carbonate (BC), and vinylenecarbonate (VC).

Preferred examples of the cyclic esters include lactones such asγ-butyrolactone (GBL).

Preferred examples of the linear carbonates include dimethyl carbonate(DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), dipropylcarbonate (DPC), and dibutyl carbonate (DBC).

Preferred examples of the ethers include cyclic ethers such astetrahydrofuran (THF), 2-methyltetrahydrofuran (2-MeTHF), and1,4-dioxane (1,4-DOX) and linear ethers such as 1,2-dimethoxyethane(DME), 1,2-diethoxyethane (DEE), and 1,2-dibutoxyethane (DBE).

In the solvent, these compounds can be used singly or in anycombinations. Moreover, one or more of other solvents such as otheresters, e.g., methyl propionate, ethyl pivalate, butyl pivalate, hexylpivalate, octyl pivalate, or dodecyl pivalate, can be used incombination.

When a cyclic carbonate and a linear carbonate is employed incombination, they are preferably employed in a volume ratio of 1:9 to9:1 (cyclic carbonate:linear carbonate), more preferably 1:4 to 1:1.

When a cyclic carbonate and an ether is employed in combination, theyare preferably employed in a volume ratio of 1:9 to 9:1 (cycliccarbonate:ether), more preferably 1:4 to 1:1.

When a cyclic carbonate and a cyclic ester is employed in combination,they are preferably employed in a volume ratio of 1:99 to 99:1 (cycliccarbonate:cyclic ester), more preferably 1:9 to 9:1, most preferably 1:4to 1:1.

Examples of the electrolytes to be incorporated into the non-aqueoussolvent include LiPF₆, LiBF₄, LiAsF₆, LiClO₄, LiOSO₂CF₃, LiN(SO₂CF₃)₂,LiN(SO₂C₂F₅)₂, LiC(SO₂CF₃)₃, LiPF₄ (CF₃)₂, LiPF₃(CF₃)₃, LiPF₃(C₂F₅)₃,LiPF₅(iso-C₃F₇), LiPF₄(iso-C₃F₇)₂, and LiBF₃(C₂F₅). The electrolytes canbe employed singly or in combination. Generally, the electrolyte can beincorporated into the non-aqueous solvent in such an amount to give anelectrolytic solution of 0.1 M to 3 M, preferably 0.5 M to 1.5 M.

A non-aqueous secondary battery of the invention comprises a positiveelectrode and a negative electrode in addition to the non-aqueouselectrolytic solution.

The positive electrode generally comprises a positive electrode activematerial and an electro-conductive binder composition.

The positive electrode active material for a lithium secondary batterypreferably is a complex metal oxide containing one metal elementselected from the group consisting of cobalt, manganese, nickel,chromium, iron, and vanadium and a lithium element. Examples of thecomplex metal oxides include LiCoO₂, LiMn₂O₄, LiNiO₂ andLiCO_(1-x)Ni_(x)O₂ (0.01<x<1).

The positive electrode active material for a lithium primary batterypreferably is an oxide of one or more metals or a calcogen compound suchas CuO, Cu₂O, Ag₂O, Ag₂CrO₄, CuS, CuSO₄, TiO₂, TiS₂, SiO₂, SnO, V₂O₅,V₆O₁₂, VO_(x), Nb₂O₅, Bi₂O₃, Bi₂Pb₂O₅, Sb₂O₃, CrO₃, Cr₂O₃, MoO₃, WO₃,SeO₂, MnO₂, Mn₂O₃, Fe₂O₃, FeO, Fe₃O₄, Ni₂O₃, NiO, CoO₃, or CoO, a sulfurcompound such a SO₂ or SOCl₂, or carbon fluoride having a formula of(CF_(x))_(n). Preferred are MnO₂, V₂O₅, and carbon fluoride.

The electro-conductive binder composition can be produced by a mixtureof an electro-conductive material such as acetylene black or carbonblack, a binder such as poly(tetrafluoroethylene) (PTFE),poly(vinylidene fluoride) (PVDF), styrene-butadiene copolymer (SBR),acrylonitrile-butadiene copolymer (NBR) or carboxymethylcellulose (CMC),and a solvent. For the preparation of a positive electrode, the mixtureis coated on a metal plate such as aluminum foil or stainless plate,dried, and pressed for molding. The molded product is then heated invacuo at a temperature of approx. 50 to 250° C. for approx. 2 hours, togive the desired positive electrode.

The negative electrode comprises a negative electrode active materialsuch as a lithium metal, a lithium alloy, carbonaceous material having agraphite-type crystalline structure which can absorb and release lithiumion, or a complex tin oxide. Examples of the carbonaceous materialsinclude thermally decomposed carbonaceous materials, cokes, graphites(e.g., artificial graphite and natural graphite), fired organic polymermaterials, and carbon fibers. Preferred are carbonaceous materialshaving a graphite-type crystalline structure in which the latticedistance of lattice surface (002), namely, d₀₀₂, is 0.34 nm (nanometer)or less, preferably 0.336 nm or less.

The negative electrode active material in the powdery form such ascarbonaceous powder is preferably used in combination with a binder suchas ethylene propylene diene terpolymer (EPEM), polytetrafluoroethylene(PTFE), poly(vinylidene fluoride) (PVDF), styrene-butadiene copolymer(SBR), acrylonitrile-butadiene copolymer (NBR) or carboxymethylcellulose(OCMC).

There are no specific limitations with respect to the structure of thenon-aqueous lithium battery of the invention. For instance, thenon-aqueous secondary battery can be a battery of coin type comprising apositive electrode, a negative electrode, and single or pluralseparators, or a cylindrical or prismatic battery comprising a positiveelectrode, a negative electrode, and a separator roll. The separator canbe a known microporous polyolefin film, woven fabric, or non-wovenfabric.

The present invention is further described by the following non-limitingexamples.

Incorporation of Dinitrile Compound into Electrolytic Solution forLithium Secondary Battery Example A-1 1) Preparation of Non-AqueousElectrolytic Solution

In a non-aqueous mixture of ethylene carbonate and methyl ethylcarbonate [EC:MEC=3:7, volume ratio] was dissolved LiPF₆ to give anon-aqueous electrolytic solution of 1 M concentration. To theelectrolytic solution was added adiponitrile in an amount of 0.01 wt. %(based on the amount of the electrolytic solution).

2) Preparation of Lithium Secondary Battery and Measurement of itsBattery Characteristics

LiCoO₂ (positive electrode active material, 90 wt. %), acetylene black(electro-conductive material, 5 wt. %), and poly(vinylidene fluoride)(binder, 5 wt. %) were mixed. To the resulting mixture was further added1-methyl-2-pyrrolidone (solvent). Thus produced positive electrodemixture was coated on aluminum foil, dried, molded under pressure, andheated to give a positive electrode.

A natural graphite (negative electrode active material, d₀₀₂=0.3354 nm,90 wt. %) and poly(vinylidene fluoride) (binder, 10 wt. %) were mixed.To the resulting mixture was further added 1-methyl-2-pyrrolidone(solvent). Thus produced negative electrode mixture was coated on copperfoil, dried, molded under pressure, and heated to give a negativeelectrode.

The positive and negative electrodes, a microporous polypropylene filmseparator, and the non-aqueous electrolytic solution were combined togive a coin-type battery (diameter: 20 mm, thickness: 3.2 mm).

The coin-type battery was charged at room temperature (20° C.) with aconstant electric current (0.8 mA, per electrode area) to reach 4.2 Vand then the charging was continued under a constant voltage of 4.2 V.In total, the charging was performed for 5 hours. Subsequently, thebattery was discharged to give a constant electric current (0.8 mA). Thedischarge was continued to give a terminal voltage of 2.7 V. Thecharge-discharge cycle was repeated 100 times.

The initial discharge capacity was 1.00 time as much as that measured ina battery using an EC/MEC (3/7) solvent mixture (containing 1M LiPF₆ butno adiponitrile) [see Comparison Example A-1].

After the 100 cycle charge-discharge procedure was complete, thedischarge capacity became 86.2% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 1.

After the evaluation on battery performances was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had the same glossy inner surface as that observed before theelectrolytic solution was placed.

Example A-2

The procedures of Example A-1 were repeated except that adiponitrile wasincorporated into the electrolytic solution in an amount of 0.05 wt. %,to prepare a coin-type battery.

The initial discharge capacity was 0.99 time as much as that measured ina battery of Comparison Example A-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became87.9% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 1.

Example A-3

The procedures of Example A-1 were repeated except that adiponitrile wasincorporated into the electrolytic solution in an amount of 0.1 wt. %,to prepare a coin-type battery.

The initial discharge capacity was 1.01 times as much as that measuredin a battery of Comparison Example A-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became89.0% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 1.

Example A-4

The procedures of Example A-1 were repeated except that adiponitrile wasincorporated into the electrolytic solution in an amount of 0.2 wt. %,to prepare a coin-type battery.

The initial discharge capacity was 1.02 times as much as that measuredin a battery of Comparison Example A-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became89.2% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 1.

After the evaluation on battery performances was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had the same glossy inner surface as that observed before theelectrolytic solution was placed.

Example A-5

The procedures of Example A-1 were repeated except that adiponitrile wasincorporated into the electrolytic solution in an amount of 0.5 wt. %,to prepare a coin-type battery.

The initial discharge capacity was 0.99 time as much as that measured ina battery of Comparison Example A-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became88.8% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 1.

Example A-6

The procedures of Example A-1 were repeated except that adiponitrile wasincorporated into the electrolytic solution in an amount of 1 wt. %, toprepare a coin-type battery.

The initial discharge capacity was 0.98 time as much as that measured ina battery of Comparison Example A-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became85.6% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 1.

Comparison Example A-1

The procedures of Example A-1 were repeated except that no adiponitrilewas incorporated into the electrolytic solution, to prepare a coin-typebattery.

After the 100 cycle charge-discharge procedure was complete, thedischarge capacity became 82.6% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 1.

After the evaluation on battery performances was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had no glossy inner surface like that observed before theelectrolytic solution was placed. Microscopic observation revealed thatthe inner surface had eroded spots.

Comparison Example A-2

The procedures of Example A-1 were repeated except that adiponitrile wasincorporated into the electrolytic solution in an amount of 13 wt. %, toprepare a coin-type battery.

The initial discharge capacity was 0.95 time as much as that measured ina battery of Comparison Example A-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became72.3% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 1.

Example A-7

The procedures of Example A-1 were repeated except that glutaronitrilewas incorporated in place of adiponitrile into the electrolytic solutionin an amount of 0.2 wt. %, to prepare a coin-type battery.

The initial discharge capacity was 1.00 time as much as that measured ina battery of Comparison Example A-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became88.9% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 1.

Comparison Example A-3: Trace of Example 6 of JP-A-9-161845

The procedures of Example A-1 were repeated except that the non-aqueoussolvent composition was replaced with a combination of glutaronitrileand dimethyl carbonate (19:81, volume ratio), to prepare a coin-typebattery.

The initial discharge capacity was 1.01 times as much as that measuredin a battery of Comparison Example A-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became64.7% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 1.

Comparison Example A-4

The procedures of Example A-1 were repeated except that adiponitrile wasreplaced with 0.2 wt. % of propionitrile, to prepare a coin-typebattery.

The initial discharge capacity was 0.96 time as much as that measured ina battery of Comparison Example A-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became82.4% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 1.

TABLE 1 Initial Discharge discharge capacity Nitrile compound capacityretention (wt. %) (R.V.) (%) Example A-1 Adiponitrile (0.01) 1.00 86.2A-2 Adiponitrile (0.05) 0.99 87.9 A-3 Adiponitrile (0.1) 1.01 89.0 A-4Adiponitrile (0.2) 1.02 89.2 A-5 Adiponitrile (0.5) 0.99 88.8 A-6Adiponitrile (1) 0.98 85.6 A-7 Glutaronitrile (0.2) 1.00 88.9 ComparisonA-1 None 1 82.6 A-2 Adiponitrile (13) 0.95 72.3 A-3 [Glutaronitrile(19)] 1.01 64.7 A-4 Propionitrile (0.2) 0.96 82.4Remarks: Comparison Example A-3 uses glutaronitrile/methyl carbonate(19:81, vol. ratio) and 1 mol/L LiPF₆. R.V. means “Relative Value”.

Example A-83 1) Preparation of Non-Aqueous Electrolytic Solution

In a non-aqueous mixture of ethylene carbonate and γ-butyrolactone[EC:GBL=3:7, volume ratio] was dissolved LiBF₄ to give a non-aqueouselectrolytic solution of 1.5 M concentration. To the electrolyticsolution were added n-butyl pivalate (separator-wetting improver) andadiponitrile in amounts of 5 wt. % and 0.2 wt. % (based on the amount ofthe electrolytic solution), respectively.

2) Preparation of Lithium Secondary Battery and Measurement of itsBattery Characteristics

The procedures of Example A-1 were repeated except that theabove-prepared electrolytic solution was used, to prepare a coin-typebattery.

The initial discharge capacity was 0.96 time as much as that measured ina battery of Comparison Example A-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became70.1% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 2.

Example A-9 1) Preparation of Non-Aqueous Electrolytic Solution

In a non-aqueous γ-butyrolactone solvent was dissolved LiBF₄ to give anon-aqueous electrolytic solution of 1.5 M concentration. To theelectrolytic solution were added n-butyl pivalate (separator-wettingimprover) and adiponitrile in amounts of 5 wt. % and 0.2 wt. % (based onthe amount of the electrolytic solution), respectively.

2) Preparation of Lithium Secondary Battery and Measurement of itsBattery Characteristics

The procedures of Example A-1 were repeated except that theabove-prepared electrolytic solution was used, to prepare a coin-typebattery.

The initial discharge capacity was 0.98 time as much as that measured ina battery of Comparison Example A-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became67.3% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 2.

Comparison Example A-5 1) Preparation of Non-Aqueous ElectrolyticSolution

In a non-aqueous mixture of ethylene carbonate and γ-butyrolactone[EC:GBL=3:7, volume ratio] was dissolved LiBF₄ to give a non-aqueouselectrolytic solution of 1.5 M concentration. To the electrolyticsolution was added n-butyl pivalate (separator-wetting improver) in anamount of 5 wt. % (based on the amount of the electrolytic solution),but added no dinitrile compound.

2) Preparation of Lithium Secondary Battery and Measurement of itsBattery Characteristics

The procedures of Example A-1 were repeated except that theabove-prepared electrolytic solution was used, to prepare a coin-typebattery.

The initial discharge capacity was 0.97 time as much as that measured ina battery of Comparison Example A-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became61.4% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 2.

Comparison Example A-6 1) Preparation of Non-Aqueous ElectrolyticSolution

In a non-aqueous γ-butyrolactone solvent was dissolved LiBF₄ to give anon-aqueous electrolytic solution of 1.5 M concentration. To theelectrolytic solution was added n-butyl pivalate (separator-wettingimprover) in an amount of 5 wt. % (based on the amount of theelectrolytic solution), but added no dinitrile compound.

2) Preparation of Lithium Secondary Battery and Measurement of itsBattery Characteristics

The procedures of Example A-1 were repeated except that theabove-prepared electrolytic solution was used, to prepare a coin-typebattery.

The initial discharge capacity was 1.00 time as much as that measured ina battery of Comparison Example A-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became59.5% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 2.

TABLE 2 Initial Discharge discharge capacity Nitrile compound capacityretention (wt. %) (R.V.) (%) Example A-8 Adiponitrile (0.2) 0.96 70.1A-9 Adiponitrile (0.2) 0.98 67.3 Comparison A-5 None 0.97 61.4 A-6 None1.00 59.5Remarks: Example A-8 and Comparison Example A-5 use EC/GBL (3:7)solvent, and Example A-9 and Comparison Example A-6 use GBL solvent.

Summary of Evaluations

The results of Examples A-1 to A-6 and Comparison Examples A-1 and A-2indicate that the use of the electrolytic solution containing anappropriate amount of a dinitrile compound gives a favorably effect to alithium secondary battery (enhancement of a discharge capacity retentionwithout lowering the initial discharge capacity after a long termcharge-discharge cycles) as compared with an electrolytic solutioncontaining no dinitrile compound. The results of Comparison Examples A-2and A-3 indicate that the use of a dinitrile compound in an excessiveamount gives an adverse effect to the battery performances. The resultsof Comparison Example A-4 indicate that the use of a mononitrilecompound in place of a dinitrile compound give almost no favorableeffect to the discharge capacity retention.

In summary, the incorporation of an appropriate amount of a dinitrilecompound into a non-aqueous electrolytic solution gives favorablyeffects to battery performances such as discharge capacity retention byforming a protective film on metallic supports of the positive andnegative electrodes and a battery case and hence keeping conductivitybetween the electrode active material and metallic support fromlowering.

Incorporation of Dinitrile Compound into Electrolytic Solution forLithium Primary Battery Example B-1 1) Preparation of Non-AqueousElectrolytic Solution

In a non-aqueous mixture of propylene carbonate and 1,2-dimethoxyethane[PC:DME=1:1, volume ratio] was dissolved LiOSO₂CF₃ to give a non-aqueouselectrolytic solution of 1.0 M concentration. To the electrolyticsolution was added adiponitrile in an amount of 0.2 wt. % (based on theamount of the electrolytic solution).

2) Preparation of Lithium Primary Battery and Measurement of its BatteryCharacteristics

MnO₂ (positive electrode active material, 85 wt. %), acetylene black(electro-conductive material, 10 wt. %), and poly(vinylidene fluoride)(binder, 5 wt. %) were mixed. To the resulting mixture was further added1-methyl-2-pyrrolidone (solvent). Thus produced positive electrodemixture was coated on aluminum foil, dried, molded under pressure, andheated to give a positive electrode.

A lithium metal foil (negative electrode material) having a thickness of0.2 mm was punched out to give a disc which was then pressed on anegative electrode collector to give a negative electrode.

The positive and negative electrodes, a microporous polypropylene filmseparator, and the non-aqueous electrolytic solution were combined togive a coin-type battery (diameter: 20 mm, thickness: 3.2 m).

The prepared coin-type battery was subjected to the following capacitytest and high temperature storage test.

[Capacity Test]

The coin-type battery was charged at room temperature (20° C.) with aconstant electric current (0.5 mA per an electrode area) to reach 3.5 V.Subsequently, the battery was discharged to give a constant electriccurrent (1.0 mA). The discharge was continued to give a terminal voltageof 2.4 V, to measure a discharge capacity.

[High Temperature Storage Test]

The coin-type battery was charged at room temperature (20° C.) with aconstant electric current (0.5 mA per electrode area) to reach 3.5 V.Subsequently, the battery was kept in a thermostat at 60° C. for 20days. Then, the battery was discharged to give a constant electriccurrent (1.0 mA). The discharge was continued to give a terminal voltageof 2.4 V, to measure a discharge capacity. A self discharge ratio in thehigh temperature storage was calculated from thus measured dischargecapacity and the discharge capacity measured at room temperature.

The discharge capacity was 1.04 time as much as that measured in abattery using a PC/DME (1/1) solvent mixture (containing 1.0 MLiOSO₂CF₃, but no adiponitrile) [see Comparison Example B-1]. Theself-discharge ratio in the high temperature storage was 6.2%.

The preparation and evaluation of the battery are summarized in Table 3.

After the high temperature storage evaluation was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had the same glossy inner surface as that observed before theelectrolytic solution was placed.

Comparison Example B-1

The procedures of Example B-1 were repeated except that no adiponitrilewas incorporated in the electrolytic solution, to prepare a coin-typebattery.

The prepared coin-type battery was subjected to the capacity test andhigh temperature storage test. The self-discharge ratio in the hightemperature storage was 10.5%.

The preparation and evaluation of the battery are summarized in Table 3.

After the high temperature storage evaluation was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had no glossy inner surface like that observed before theelectrolytic solution was placed. Microscopic observation revealed thatthe inner surface had eroded spots.

Comparison Example B-2

The procedures of Example B-1 were repeated except that adiponitrile wasincorporated into the electrolytic solution in an amount of 13 wt. %, toprepare a coin-type battery.

The prepared coin-type battery was subjected to the capacity test andhigh temperature storage test.

The discharge capacity was 0.92 time as much as that measured inComparison Example B-1. The self-discharge ratio in the high temperaturestorage was 22.8%.

The preparation and evaluation of the battery are summarized in Table 3.

After the high temperature storage evaluation was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had the same glossy inner surface as that observed before theelectrolytic solution was placed.

TABLE 3 High temperature Discharge self-discharge Nitrile compoundcapacity ratio (wt. %) (R.V.) (%) Example B-1 Adiponitrile (0.2) 1.046.2 Comparison B-1 None 1 10.5 B-2 Adiponitrile (13) 0.92 22.8

Incorporation of Nitrile Compound and S═O Group-Containing Compound intoElectrolytic Solution for Lithium Secondary Battery Example C-1 1)Preparation of Non-Aqueous Electrolytic Solution

In a non-aqueous mixture of ethylene carbonate, vinylene carbonate, andmethyl ethyl carbonate [EC:VC:MEC=28:2:70, volume ratio] was dissolvedLiPF₆ to give a non-aqueous electrolytic solution of 1 M concentration.To the electrolytic solution were added 1,4-dicyanobenzene and ethylenesulfite in amounts of 2 wt. % and 2 wt. % (based on the amount of theelectrolytic solution), respectively.

2) Preparation of Lithium Secondary Battery and Measurement of itsBattery Characteristics

A coin-type battery was prepared using the above-obtained electrolyticsolution in the same manner as described in Example A-1, and the batteryperformances were measured in the same manner.

The initial discharge capacity was 1.00 time as much as that measured ina battery using an EC/VC/MEC(28/2/70) solvent mixture (containing 1MLiPF₆ but neither 1,4-dicyanobenzene nor ethylene sulfite) [seeComparison Example C-1].

After the 100 cycle charge-discharge procedure was complete, thedischarge capacity became 88.9% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 4.

After the evaluation on battery performances was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had the same glossy inner surface as that observed before theelectrolytic solution was placed.

Comparison Example C-1

The procedures of Example C-1 were repeated except that neither1,4-dicyanobenzene nor ethylene sulfite was incorporated into theelectrolytic solution, to prepare a coin-type battery.

After the 100 cycle charge-discharge procedure was complete, thedischarge capacity became 83.7% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 4.

After the evaluation on battery performances was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had no glossy inner surface like that observed before theelectrolytic solution was placed. Microscopic observation revealed thatthe inner surface had eroded spots.

Example C-2

The procedures of Example C-1 were repeated except that1,4-dicyanobenzene was replaced with adiponitrile, to prepare acoin-type battery.

The initial discharge capacity was 1.00 time as much as that measured ina battery of Comparison Example C-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became90.2% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 4.

After the evaluation on battery performances was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had the same glossy inner surface as that observed before theelectrolytic solution was placed.

Example C-3

The procedures of Example C-2 were repeated except that 3M of LiPF₆ wasreplaced with a combination of 0.9M of LiPF₆ and 0.1M of LiN(SO₂CF₃)₂,to prepare a coin-type battery.

After the 100 cycle charge-discharge procedure was complete, thedischarge capacity became 89.4% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 4.

After the evaluation on battery performances was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had the same glossy inner surface as that observed before theelectrolytic solution was placed.

Example C-4

The procedures of Example C-2 were repeated except that 1M of LiPF₆ wasreplaced with a combination of 0.9M of LiPF₆ and 0.1M of LiBF₄, toprepare a coin-type battery.

After the 100 cycle charge-discharge procedure was complete, thedischarge capacity became 89.7% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 4.

After the evaluation on battery performances was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had the same glossy inner surface as that observed before theelectrolytic solution was placed.

Example C-5

The procedures of Example C-2 were repeated except that ethylene sulfitewas replaced with 1,3-propanesultone, to prepare a coin-type battery.

The initial discharge capacity was 1.00 time as much as that measured ina battery of Comparison Example C-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became89.8% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 4.

After the evaluation on battery performances was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had the same glossy inner surface as that observed before theelectrolytic solution was placed.

Example C-6

The procedures of Example C-2 were repeated except that 2 wt. % ofethylene sulfite was replaced with 0.3 wt. % of divinylsulfone, toprepare a coin-type battery.

The initial discharge capacity was 1.00 time as much as that measured ina battery of Comparison Example C-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became89.4% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 4.

After the evaluation on battery performances was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had the same glossy inner surface as that observed before theelectrolytic solution was placed.

Example C-7

The procedures of Example C-2 were repeated except that 2 wt. % ofethylene sulfite was replaced with 0.5 wt. % of propargylmethanesulfonate, to prepare a coin-type battery.

The initial discharge capacity was 1.00 time as much as that measured ina battery of Comparison Example C-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became89.3% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 4.

After the evaluation on battery performances was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had the same glossy inner surface as that observed before theelectrolytic solution was placed.

Comparison Example C-2

The procedures of Example C-2 were repeated except that no1,4-cyanobenzene was used, to prepare a coin-type battery.

The initial discharge capacity was 1.00 time as much as that measured ina battery of Comparison Example C-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became84.2% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 4.

After the evaluation on battery performances was complete, the batterywas disassembled to observe the inner surface of the battery case. Thecase had no glossy inner surface like that observed before theelectrolytic solution was placed. Microscopic observation revealed thatthe inner surface had eroded spots.

Example C-8

The procedures of Example C-1 were repeated except that1,4-dicyanobenzene was replaced with propionitrile, to prepare acoin-type battery.

The initial discharge capacity was 1.00 time as much as that measured ina battery of Comparison Example C-1. After the 100 cyclecharge-discharge procedure was complete, the discharge capacity became88.2% of the initial discharge capacity.

The preparation and evaluation of the battery are summarized in Table 4.

TABLE 4 Electrolytic solution Initial Discharge Nitrile compound(wt. %)discharge capacity SO₂-containing capacity retention Examplecompound(wt. %) (R.V.) (%) C-1 1M LiPF₆ in EC/VC/MEC 1.00 88.91,4-Dicyanobenzene (2) Ethylene sulfite (2) C-2 1M LiPF₆ in EC/VC/MEC1.00 90.2 Adiponitrile (2) Ethylene sulfite (2) C-3 0.9M LiPF₆ + 0.1MLiN(SO₂CF₃)₂ 1.00 89.4 in EC/VC/MEC Adiponitrile (2) Ethylene sulfite(2) C-4 0.9M LiPF₆ + 0.1M LiBF₄ in 1.00 89.7 EC/VC/MEC Adiponitrile (2)Ethylene sulfite (2) C-5 1M LiPF₆ in EC/VC/MEC 1.00 89.8 Adiponitrile(2) 1,3-Propanesultone (2) C-6 1M LiPF₆ in EC/VC/MEC 1.00 89.4Adiponitrile (2) Divinylsulfone (0.3) C-7 1M LiPF₆ in EC/VC/MEC 1.0089.3 Adiponitrile (2) Propargyl methane- sulfonate (0.5) C-8 1M LiPF₆ inEC/VC/MEC 1.00 88.2 Propionitrile (2) Ethylene sulfite (2) Comparison 1MLiPF₆ in EC/VC/MEC 1.00 83.7 C-1 None None Comparison 1M LiPF₆ inEC/VC/MEC 1.00 84.2 C-2 None Ethylene sulfite (2)

1: A non-aqueous electrolytic solution, comprising: a non-aqueoussolvent comprising from 30 to 50 volume percent of a cyclic carbonate,based on a total volume of the non-aqueous solvent; from 0.5 to 1.5 M ofat least one electrolyte selected from the group consisting of LiPF₆ andLiBF₄; from 0.01 to 3% by weight of adiponitrile, based on a totalweight of the non-aqueous electrolytic solution; and from 0.2 to 3% byweight of at least one S═O group-containing compound selected from thegroup consisting of ethylenesulfite, 1,3-propanesultone, divinylsulfone,and propargyl methanesulfonate, based on a total weight of thenon-aqueous electrolytic solution. 2: The non-aqueous electrolyticsolution of claim 1, wherein the cyclic carbonate comprises ethylenecarbonate. 3: The non-aqueous electrolytic solution of claim 1, whereinthe electrolyte comprises LiPF₆. 4: The non-aqueous electrolyticsolution of claim 1, wherein the S═O group-containing compound comprises1,3-propanesultone. 5: A lithium secondary battery, comprising: apositive electrode; a negative electrode comprising a carbonaceousmaterial of graphite crystal structure having a lattice distance oflattice surface (002) of 0.34 nanometer or less; and the non-aqueouselectrolytic solution of claim
 1. 6: The lithium secondary battery ofclaim 5, wherein the positive electrode comprises a lithium element anda complex metal oxide comprising one metal element selected from thegroup consisting of cobalt, manganese, nickel, chromium, iron, andvanadium. 7: The lithium secondary battery of claim 6, wherein thecomplex metal oxide comprises at least one selected from the groupconsisting of LiCoO₂, LiMn₂O₄, LiNiO₂ and LiCO_(1-x)Ni_(x)O₂(0.01<x<1).8: A lithium secondary battery, comprising: a positive electrode, anegative electrode comprising a carbonaceous material of graphitecrystal structure having a lattice distance of lattice surface (002) of0.34 nanometer or less; and the non-aqueous electrolytic solution ofclaim
 2. 9: The lithium secondary battery of claim 8, wherein thepositive electrode comprises a lithium element and a complex metal oxidecomprising one metal element selected from the group consisting ofcobalt, manganese, nickel, chromium, iron, and vanadium. 10: The lithiumsecondary battery of claim 9, wherein the complex metal oxide comprisesat least one selected from the group consisting of LiCoO₂, LiMn₂O₄,LiNiO₂ and LiCO_(1-x)Ni_(x)O₂(0.01<x<1). 11: A lithium secondarybattery, comprising: a positive electrode; a negative electrodecomprising a carbonaceous material of graphite crystal structure havinga lattice distance of lattice surface (002) of 0.34 nanometer or less;and the non-aqueous electrolytic solution of claim
 3. 12: The lithiumsecondary battery of claim 11, wherein the positive electrode comprisesa lithium element and a complex metal oxide comprising one metal elementselected from the group consisting of cobalt, manganese, nickel,chromium, iron, and vanadium. 13: The lithium secondary battery of claim12, wherein the complex metal oxide comprises at least one selected fromthe group consisting of LiCoO₂, LiMn₂O₄, LiNiO₂ andLiCO_(1-x)Ni_(x)O₂(0.01<x<1). 14: A lithium secondary battery,comprising: a positive electrode; a negative electrode comprising acarbonaceous material of graphite crystal structure having a latticedistance of lattice surface (002) of 0.34 nanometer or less; and thenon-aqueous electrolytic solution of claim
 4. 15: The lithium secondarybattery of claim 14, wherein the positive electrode comprises a lithiumelement and a complex metal oxide comprising one metal element selectedfrom the group consisting of cobalt, manganese, nickel, chromium, iron,and vanadium. 16: The lithium secondary battery of claim 15, wherein thecomplex metal oxide comprises at least one selected from the groupconsisting of LiCoO₂, LiMn₂O₄, LiNiO₂ and LiCO_(1-x)Ni_(x)O₂ (0.01<x<1).